Description: In a simple graph, the number of neighbors of a vertex is the degree of
this vertex. This theorem does not hold for (simple) pseudographs,
because a vertex connected with itself only by a loop has no neighbors,
see uspgrloopnb0 , but degree 2, see uspgrloopvd2 . And it does not
hold for multigraphs, because a vertex connected with only one other
vertex by two edges has one neighbor, see umgr2v2enb1 , but also
degree 2, see umgr2v2evd2 . (Contributed by Alexander van der Vekens, 17-Dec-2017)(Revised by AV, 15-Dec-2020)(Proof shortened by AV, 5-May-2021)